The invention pertains generally to coating a surface and more particularly to melt-coating a surface with a staticly charged powder.
Melt-coating pertains to coating a surface by either melting a powder in contact with the surface or by melting the surface and embedding a powder, e.g., refractory ceramic powder in the molten surface to form a composite surface. In both cases, energy, e.g., a laser beam, radiant heat, electron beam or proton beam is directed on a powder and surface to cause one or both to melt. Several general problems exist with this technique.
One problem is a large reflection by some of the powders, e.g., tantalum carbide, causing much wasted energy and longer processing times. Large reflections make additional requirements on the equipment to shield the reflected energy.
Another problem is degradation of the surface before the powder has been adequately heated. Even if the surface is being melted, this problem exists because the composite layer often has improved properties if the powder has a high temperature at the time the molten surface solidifies. Consequently, a choice has to be made between either inadequately heating the powder or tolerating degradation of the surface. Of course, many melt-coating applications are not possible or practical on account of this problem.
A coating is often needed only on a certain portion of a surface, requiring the non-coated portions to be shielded or masked. These techniques increase the cost and complexity of processing and the chance for contamination of the coating.
Presently, static-charging a powder is being used to facilitate the movement of large quantities of powder which tend to agglomerate. An example of this application is given in U.S. Pat. No. 2,462,487 by Griffith et al. An opposite application of static charging a powder is given in U.S Pat. No. 2,896,263 by Frederick et al., wherein the bulk density of a powder is reduced and agglomeration is increased by subjecting the powder to an electrostatic charge that is opposite to the natural electrostatic charge. In U.S. Pat. No. 3,598,566 by Yoshi Inoue, static charging is used to improve purity and to clean the surface of the powder by removing oxides, moisture and other impurities. Static charging is used to improve the sintering of powder in Japanese Pat. No. 45-31044. The probable mechanism for the improved sintering is the removal of oxides and other impurities.